It works for few people on jellybean, system structure is different in cm10 room. Still, it works for some. I use gscript lite with root and two scripts for switching back and forth between UMS and MTP. I'll have to find the link to directions and scripts again on xda.
EDIT: Here are the instructions to enable and disable mass storage. http://forum.xda-developers.com/showthread.php?p=29063080
Also, here is how to enable the Samsung 50GB dropbox storage for two years offer that Verizon blocked. http://forum.xda-developers.com/showthread.php?p=30238829